The present invention is in the technical field of measurement of electric parameters.
More particularly, the present invention is in the technical field of voltage phase angle measurements on an alternating current (a.c.) power grid.
The voltage and current on an a.c. power grid have a fundamental frequency, often one of the following: 50 Hertz, 60 Hertz, or 400 Hertz. For many applications, it can be useful to measure the phase angle of the voltage fundamental frequency, or the phase angle of the current fundamental frequency, or both. Such a measurement can be made either relative to the phase angle at a different physical location, or relative to a fixed time base such as that provided by the Global Positioning System. The measured fundamental angle can be combined with the measured fundamental magnitude to form a fundamental phasor measurement.
Phasor measurements can be equivalently expressed in polar coordinates, as an angle and a magnitude; or they can be expressed in Cartesian coordinates, typically for a.c. systems as a real and an imaginary component; or they can be expressed as a vector on a rotating Cartesian coordinate system that completes one rotation per nominal fundamental cycle; or they can be expressed in any other mathematically-equivalent way.
One known phasor application for a.c. grids, well known to those familiar with the art, is the synchrophasor application, in which the voltage phasor, current phasor, or both are examined simultaneously at two or more separate physical locations on an a.c. grid that connects those two or more locations. In this known application, the difference between phasors at the two separate physical locations may, for example, provide useful information about the power flow between those two locations.
Typically, synchrophasor applications have been applied to high-voltage power transmission systems, even if the measurements themselves are made on local low-voltage locations.
In those synchrophasor applications on transmission systems, the difference in phase angle between two separate physical locations can often be tens of degrees or more, and detecting interesting phenomena rarely requires a resolution better than about half a degree. Indeed, the IEEE Standard C37.118 (2011) for synchrophasor measurements only requires a Total Vector Error of 1% or better, which corresponds to approximately ±0.5°.
In our Department of Energy ARPA-E Project DE-AR0000340, titled “Micro-Synchrophasors for Distribution Systems,” we have been investigating the application of synchrophasor measurements to medium-voltage distribution grids, as opposed to the traditional application to high-voltage transmission grids. Due to smaller inductances and shorter distances on distribution grids compared to transmission grids, the phase angle changes during interesting phenomena on distribution grids are much smaller. We have determined that, for distribution grid applications, a angular resolution for voltage phasors and current phasors of ±0.015° could be useful.
Transmission grids generally operate at 100,000 volts or higher, and distribution grids generally operate at 1,000 volts to 100,000 volts. As is well known in the art, to measure a.c. voltage on these grids it is necessary to proportionally reduce the a.c. voltage to an acceptable level for electronic devices, which conventionally measure signals that are less than 1,000 volts.
Typically, this proportional voltage reduction is done with transformers. One commonly-available type of transformer, which we will call a distribution transformer, is intended to supply a significant amount of power to a load, such as a group of residences or a factory, but can also be used for making phasor measurements. The medium-voltage primary winding of distribution transformers is connected to the distribution grid; the low-voltage secondary winding delivers power to consumers, and is at a level that can be measured by electronic devices.
In general, we are interested in making phasor measurements that indicate the voltage magnitudes and angles on the distribution conductors, but as a practical matter we instead measure the voltage phasors on the secondary windings of a transformer.
Consequently, any phase angle shifts that occur inside the transformer, between the primary winding and the secondary winding, will affect the accuracy and resolution of a voltage phasor measurement.
Prior to the present invention, it was believed by those familiar with the art that high-precision medium-voltage phasor measurements, using generally available distribution transformers, would be impossible. It is well known to those familiar with the art that the voltage phasor on the secondary winding of distribution transformers, where the measurement would take place, is strongly affected by the uncontrolled loads that are supplied by the secondary winding of a distribution transformer.